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Spacetime Explained
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0:00
[INTRO MUSIC] Today's episode is about space, time, and the nature of reality. My name is Gabe, and this time, it really is "Space Time." [THEME MUSIC]
0:18
If you pay attention, this episode is going to blow your mind, so we're going to take it slow. What is spacetime, exactly? Before I can answer that, I need you to do something for me. Give up your intuitions about how time and space work. At first, your brain might resist and hold onto those intuitions for dear life. Don't worry. That's normal. This is challenging for everyone, even Einstein. Ready? OK. Spacetime refers to whichever external reality underlies
0:49
our collective experiences of the space between things and the time between events. Why can't space and time just be a reality? Why add spacetime as an extra concept? Here's why. Suppose two observers are moving relative to each other, and particles count as observers. Fact-- those observers don't agree about how much time passes between events. Fact-- they don't fully agree on how much space there is between things at any given moment. Fact-- they don't even agree on the chronological order
1:24
of all events. And yet, each observer measures things properly and is entirely consistent, which means neither of them is wrong. Now that sounds absurd, but it's true. Plenty of other resources, some of which we link to in the description, discuss these discrepancies and the experimental evidence for them. For today, we'll just take them at face value and focus on what they imply about the nature of reality. Because if you think about it, some of the implications are staggering.
1:52
Take this disagreement about sequence of events, for instance. It is severe. If two observers can't agree on the sequence of events, it means that at present, someone's past is in someone else's future. Now for nearby events, the effect is microscopic, but so what? Any disagreement means that there is no universal division of events into past, present, and future, which opens major philosophical cans of worms for things like free will and our belief that we can change the future.
2:21
So is everyone's experience of the universe entirely subjective? Or phrased another way, if time and space as we usually conceive of them aren't part of objective reality, then what is? Causality. Let me explain. A good starting point for objective reality is universal agreement. And as luck would have it, all observers do agree about this thing. It's called the spacetime interval, or spacetime separation between two events. Even though two observers in relative motion
2:55
will measure different distances and different elapsed times between the same two events, they always agree about the spacetime interval between those events. Now if everyone agrees about spacetime intervals, they must signify something. But what? We'll notice that since it involves subtraction, a spacetime interval can be positive, zero, or negative. When it's positive, nothing can get from one event to the other, and there are always observers who disagree about which one happens first.
3:27
When it's zero or negative, signals or things can get from one event to the other and everyone agrees on their sequence. So it appears that the spacetime interval between events A and B tells you whether A can influence B. In other words, even though we can't agree about past, present, future, time, or distance, we all appear to agree about causality. Now that may seem counterintuitive. Normally we think that time is responsible for causality. But actually it's the other way around.
3:59
To the extent that we agree about temporal anything, it's only because of causality. Causality is what's real. So what does causality have to do with spacetime? As it turns out, everything. See, shortly after relativity first came out, a former math professor of Einstein's named Hermann Minkowski noticed that the spacetime interval resembles a weird version of a distance formula in what's called a non-Euclidean space. So he proposed the following radical idea--
4:28
maybe reality is not a three dimensional space that evolves in time. Instead, it's a four dimensional non-Euclidean mathematical space that's just there. No evolution. No time. That 4D mathematical space is spacetime. Its points correspond to events. All events, everywhere, ever. And in this view, only things that correspond to geometric relations in that 4D space are objectively real, like for instance, causal relations. They correspond to spacetime intervals,
5:03
which are geometric relations, a non-Euclidean version of the distances between points. In contrast, our experiences and measurements of time and space don't correspond to anything, per se. They're more like the XY grid we use in math class, useful for talking about the board, but arbitrary and inherently meaningless. The board, its points and geometric facts, are simply there whether we put axis on that board or not. So are you objectively real? Well, kind of.
5:31
If you are the sequence of all the events of which you were present, then you are a geometric object in spacetime, a line segment joining the points representing the events of your birth and your death. Do you move along that line segment? No no. You are the line segment. There's no motion through spacetime. It's not this kind of space. It's tense-less. And your future isn't merely predetermined. It already exists. There's some zen in trying to express what spacetime
5:59
is without misleading you, but I think the following gets the flavor right. Imagine we're are all reading a flip book made of graph paper. We agree on the events of the story, but we don't agree where they happen on the page, on how many pages there are between events, or even on the order of some of those events. And yet, we're all reading the same book, only there's no graph on the paper, there are no pages, and there is no book. All of that is just an imposition
6:27
our brains make in order to perceive whatever "it" is. So why do we perceive reality in such a vividly spatial and temporal way? Good question. No one really knows. So have I told you all there is to know about spacetime? No, far from it. All of this has just been a loose introduction to what's called a flat spacetime. Once general relativity entered the mix, we'll find that there are many possible spacetimes with different geometries, making it hard to ascertain which one this is.
6:57
But we got to crawl before we walk. We will get to that fun stuff eventually though, so subscribe. And as always, the comments are for your questions. I'll do my best to answer them at the next causally-connected point of spacetime. Last week, we asked whether NASA could start a zombie apocalypse. You guys, as usual, had a lot to say. Daniel Jenkins commented that a space-based zombie outbreak assumes that a more virulent organism would actually spread better.
7:20
First of all, that assumption is unnecessary. It's enough for the bug to just be more harmful and harder fight off with your space-depressed immune system. But second, as Nicholas Garrison pointed out, germs do spread more easily space capsules for a variety of reasons, including the fact that the gunk in sneezes and coughs just hangs there instead of falling. Pretty nasty. Joe G.P. and Dikasad2 both asked what it is about space exactly that enhances virulence?
7:42
Is it the low gravity or the radiation? What is it? Well, the authors of salmonella study theorised that the signal to bacteria in microgravity might be lower shear forces on their cell membranes from the surfaces and fluids that surround it. But based on the journal articles I read, that's only one of several suggestions and the jury is still out. We just don't know. Joey Broda and McKnowledge1000 both asked why human gene expression isn't altered in orbit if bacterial gene expression is.
8:05
That's a great question, but I'm not a microbiologist. Maybe human cells do change. I don't know. It's a great question, but I have no idea. DHGameStudios said that it's a shame we put a zombie tag on this video. What do you mean? Is it a shame that "Sesame Street" teaches reading and arithmetic with a vampire and a canary with acromegaly? Have some fun, man. Lighten up. Zombies are the best. Finally, At-Bristol Science Centre really wants to see the Curiosity rover battling
8:25
martian zombies in a movie. Yeah, you do. You know who else does? Bjs301's kids and their friends, because I'm pretty sure they understand that even though zombies aren't real, they're still super awesome. Finally, quick announcement. The PBS Digital Studios Network has been nominated for a Webby award in the science and education category. We'd appreciate your support and your vote at webbyawards.com. You can check out the link in the description. [THEME MUSIC PLAYING]
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